Inks for a ballpoint pen include usually a low-viscosity water based ink for a ballpoint pen which contains a solvent comprising a water based solvent such as water and has an ink viscosity of 10 mPa.s or less and an oil based ink which contains a solvent comprising an oil based solvent such as a mineral oil, a polyhydric alcohol, a fatty acid and Cellosolve and has an ink viscosity of 1000 to 20000 mPa.s.
A ballpoint pen using an oil based ink for a ballpoint pen has such structure that the ink adhered on a ball at a pen tip through an ink reservoir having a small diameter is transferred onto a paper surface by rotation of the ball and only the transferred portion of the ink is fed to the ball from the reservoir.
A ballpoint pen using a water based ink for a ballpoint pen has such structure that the ink is fed to a ball surface and a paper surface by virtue of capillary action of a feed prepared by binding fine fibers.
While the foregoing water based ink for a ballpoint pen and the oil based ink for a ballpoint pen each have excellent advantages, they also have various problems, respectively. For example, since the water based ink for a ballpoint pen has a low viscosity, the ink is fed by means of making use of a principle of capillary action, and a ballpoint pen using this allows the capillary action to work on the contact point thereof to feed the ink only if the point portion of the ballpoint pen is simply brought into contact with paper and makes it possible to write fine lines on a paper surface without applying so higher writing pressure, so that splitting, starving and blobbing scarcely take place. On the contrary, storing the ink directly in the ink reservoir allows the ink to seep due to vibrations, impacts and a rise in a open air temperature to make the amount of the ink to be fed to the ball unstable. Accordingly, complicated structure having a feed prepared by binding fine fibers is required. Further, involved is the problem that it is difficult to check the remaining amount of the ink.
On the other hand, a ballpoint pen using an oil based ink for a ballpoint pen is characterized by that because of a high viscosity of the ink, blobbing of the ink from the pen point can be prevented and the ink can be stored directly in the ink reservoir having a small diameter, so that the structure of the ballpoint pen can be simplified and that the remaining amount of the ink can be checked by using a transparent material for the ink reservoir. On the other hand, there are involved the problems that since the ink is transferred only on a paper surface contacted with the rotating ball, splitting and starving are liable to be caused if the ball rotates unstably and that since the ink hardly penetrates into the paper surface, blobbing which causes stain with the untransferred ink is apt to be caused.
In recent years, in order to solve such problems, provided are water based inks for a ballpoint pen in which a gelatinizer or a water-soluble paste are added to impart a specific viscosity characteristic (hereinafter referred to as a pseudoplastic water based ink).
A ballpoint pen using this pseudoplastic water based ink is reduced in an ink viscosity because shear force is applied to the ink due to the rotation of the ball at the tip point at the time of writing, and can write as smoothly as a ballpoint pens using a water based ink and make fine lines on the paper surface. Further, blobbing of the ink from the pen point can be prevented because the ink has a high viscosity when the pen is not used for writing. The ballpoint pen using this pseudoplastic water based ink can store the ink directly into the ink reservoir and simplify the structure thereof, and the use of a transparent material for the ink reservoir makes it possible to check the remaining amount of the ink.
Thus, the use of the pseudoplastic water based ink for a ballpoint pen provides a writing instrument having both characteristics of a ballpoint pen using a water based ink and a ballpoint pen using an oil based ink.
A water based ink contains water as a principal solvent and therefore has the defect that if water contained therein evaporated, dissolved substances and mixtures contained in the ink are concentrated, deposited, dried and solidified at the point of the writing instrument to cause clogging and a rise in the viscosity of the ink and bring about troubles in writing. Accordingly, it has so far been proposed to add a less volatile agent for solubilizing a dye or a less volatile aid for dissolving a dye such as urea, thiourea, polyhydric alcohols or derivatives thereof, tetrahydrofurfuryl alcohol, 4-methoxy-4-methylpentane-2-one and ethylene oxide adducts of p-toluenesulfonamide and thiodiethanol, alkanolamine, and sorbitol.
However, inks blended with the various additives described above have not been satisfactory for writing instruments because of the reasons that the drying resistant effect is insufficient and the additives themselves have toxicity and therefore are not suitable as additives and that the viscosities of the inks grow large to bring about inferior follow-up of the inks.
Thus, an object of the present invention is to provide a water based ink for a writing instrument which has an excellent drying resistance and is safe and in which the viscosity does not rise. In particular, a water based ink which is suitable as a pseudoplastic water based ink for a ballpoint pen is provided.